Relay stations in optical transmission systems included in optical communication networks have adopted optical amplifiers that amplify optical signals as they are instead of using regenerative relaying involving photoelectric conversion so as to support faster (wider-bandwidth) optical signals. Optical amplifiers commonly used nowadays include those using rare-earth-doped optical fibers as amplifying mediums. In particular, erbium-doped fiber amplifiers (EDFAs) using erbium-doped fibers (EDFs) as amplifying mediums have mainly been used.
Since recent networks require a longer relay distance, higher gains are required for optical amplifiers of relay stations. When EDFAs are used, excellent amplification can be achieved by using serially connected (cascaded) EDFAs with two stages rather than EDFAs with single stages with consideration of noise figure (NF). FIG. 7 illustrates an optical amplifier including an EDFA 1 serving as a first optical amplifier and an EDFA 2 serving as a second optical amplifier serially connected to each other with an optical isolator 3 for preventing loop oscillation interposed therebetween.
The first EDFA 1 is of the forward pumping type, and includes a first EDF 1a, a first excitation light source 1b that generates excitation light, and a first optical multiplexer 1c that is disposed upstream of the first EDF 1a and multiplexes input light and excitation light generated by the first excitation light source 1b so as to supply the resultant light to the first EDF 1a. Moreover, a second EDFA 2 is of the same forward pumping type, and includes a second EDF 2a, a second excitation light source 2b that generates excitation light, and a second optical multiplexer 2c that is disposed upstream of the second EDF 2a and multiplexes input light and excitation light generated by the second excitation light source 2b so as to supply the resultant light to the second EDF 2a. 
In the optical amplifier illustrated in FIG. 7, signal light is input to the first EDF 1 through the first optical multiplexer 1c first. Excitation light is also supplied from the first excitation light source 1b to the first EDF 1a via the first optical multiplexer 1c, and the signal light input to the first EDF 1a is amplified by stimulated emission from erbium excited by the excitation light. The signal light amplified by and output from the first EDFA 1 is input to the second EDF 2a, to which excitation light is supplied from the excitation light source 2b, through an optical isolator 3 and the second optical multiplexer 2c, and is amplified in a manner similar to that in the first EDF 1a. The optical isolator 3 transmits light in only one direction from the first EDFA 1 to the second EDFA 2. With this, the isolator prevents a resonator structure from being formed, the structure having connecting points of an optical path using, for example, optical connectors serving as reflective ends at an input port IN and an output port OUT of the optical amplifier, and prevents loop oscillation of the optical amplifier.
When optical signals are amplified and relayed using the optical amplifier illustrated in FIG. 7, polarization dependent gain (PDG) occurs due to polarization hole burning (PHB) that arises in the EDFs 1a and 2a of the first and second EDFAs 1 and 2, respectively. The effect of polarization dependent gain may accumulate and may have an adverse effect when a system includes a plurality of relay stations using the optical amplifier illustrated in FIG. 7 on transmission paths thereof. For example, when signal light in the C band (approximately from 1,528 nm to 1,565 nm) is amplified and relayed, the optical signal-to-noise ratio (OSNR) of signal components in a short wavelength region in the C band is often measurably reduced.
Polarization hole burning is a phenomenon that causes the gain of signal light input to EDFs to vary in accordance with the polarization state of excitation light and the signal light input to the EDFs (Shoichi Sudo, Erbium-doped optical fiber amplifier; Optronics Co., Ltd.: Tokyo, 1999; pp 59-61). When signal light with a high intensity and a high degree of polarization (DOP) is input to EDFs, gain of light components in a polarization direction parallel to the polarization direction of the signal light is reduced due to polarization hole burning. Variations in gain in the EDFs also affect amplified spontaneous emission (ASE) light arising inside the EDFs in addition to the signal light. ASE light is not polarized, and includes polarized components parallel to the polarization direction of the signal light and those perpendiculars to the polarization direction. Therefore, only the polarized components parallel to the signal light among those in the ASE light are affected by variations in gain caused by the polarization hole burning. That is, the polarization hole burning causes a reduction in gain of the signal light and a reduction in gain of the polarized components parallel to the signal light among those in the ASE light while the gain of the polarized components perpendicular to the signal light is not reduced. Therefore, a difference between the gain of polarized components parallel to the signal light with a high degree of polarization and the gain of polarized components perpendicular to the signal light among those in the ASE light arising in the EDFs serves as a polarization dependent gain. This reduces the OSNR of the output light after being amplified compared with the case without polarization hole burning as a result of a relative increase in the proportion of the polarized components perpendicular to the signal light among those in the ASE light. That is, when signal light in a short wavelength region in the C band has a high intensity and a high degree of polarization, the light is affected by the polarization dependent gain caused by the polarization hole burning, and the OSNR thereof after amplification is reduced.
Herein, the polarization dependent gain caused by the polarization hole burning depends on the degree of polarization of the light in the EDFs, and is suppressed as the degree of polarization is reduced (For example, see Bruere F. Measurement of polarization-dependent gain in EDFAs against input degree of polarization and gain compression; Electron. Lett. 1995, 31, No. 5, pp 401-403). The term “degree of polarization” refers to a ratio of the light power of completely polarized components to total light power at a specific wavelength. When the degree of polarization is zero, it refers to a non-polarized state, and when it is one, it refers to a completely polarized state.
In FIG. 7, input light O1 including signal light S in a short wavelength region in the C band and noise light N1 with wavelengths over the entire C band may be input to the first EDFA 1. In this case, the input light O1 before being amplified has an OSNR depending on the power of the signal component with the wavelength of the signal light and the power of the noise component.
When the degree of polarization of the signal light S included in the input light O1 is high, the signal light is affected by the polarization dependent gain caused by the above-described polarization hole burning in the first EDFA 1. As a result, output light O2 after amplification by the first EDFA 1 has a higher proportion of a noise component N2 with wavelengths, in particular, adjacent to that of the signal light S, and the OSNR of the signal light S is reduced.
The output light O2 after amplification by the first EDFA 1 is subsequently input to the second EDFA 2, and further amplified by the second EDFA 2. The input light O2 is also affected by the polarization dependent gain caused by the polarization hole burning in the second EDFA 2. Therefore, output light O3 after amplification by the second EDFA 2 has a still higher proportion of a noise component N3 with wavelengths adjacent to that of the signal light S, and the OSNR of the signal light S is further reduced.
Although an optical amplifier of the forward pumping type is illustrated in FIG. 7, the OSNR may be similarly reduced even with an optical amplifier of the bidirectional pumping type.
For example, Japanese Unexamined Patent Application Publication No. 2003-315739 describes a technology for passing transmission light through a polarization scrambler in a transmitting station and sending the transmission light to a transmission path so that the transmission light is made non-polarized in order to reduce the degree of polarization of the input light to zero with consideration of polarization dependent gain in EDFAs.
When a polarization scrambler is used, the polarization scrambler needs to be controlled in synchronization with, for example, an optical modulator that generates signal light as described in Japanese Unexamined Patent Application Publication No. 2003-315739. However, the control of the polarization scrambler becomes difficult as the speed of signal light is increased, and presents problems for practical application. In addition, this leads to an increase in costs.
In view of the above-described background, apparatuses and methods for suppressing polarization dependent gain without using polarization scramblers are required.
Herein, a structure for suppressing polarization dependent gain of an optical amplifier, including a first optical amplifier and a second optical amplifier serially connected to each other on an optical path between an input port and an output port with an optical isolator interposed therebetween, will be described.